Science: Holy Grail inside his head: Francis Crick, unraveller of DNA, has now set out in search of the soul, writes Steve Connor

The human soul has been a Holy Grail for theologians, philosophers and assorted mystics down the centuries. Now the British half of one of science's better known double acts - Francis Crick of the Watson and Crick duo - has stepped into the arena with a new book detailing how he intends to continue the quest scientifically by looking in the most obvious place: inside our heads.

Crick, who has lived in California since 1976, is expounding his ideas on a lecture tour of Britain - he comes to London this week. The theme will be his 'astonishing hypothesis' that human joys, sorrows, memories, ambitions, sense of personal identity and free will 'are in fact no more than the behaviour of a vast assembly of nerve cells and their associated molecules'. So much for the Roman Catholic definition of the soul - a living being without a body, having reason and free will.

One of the more curious of Crick's ideas, which forms a postscript to his book, is that free will has a physical home in the head. It comprises a tiny bundle of cells that resides on the inside, top surface at the front of the brain. Without it we would be devoid of that essential ingredient of humanity - the ability to think, plan ahead and make rational decisions.

Crick is not short of this cranial commodity himself. He and James Watson unravelled one of the greatest mysteries of the 20th century - the double helix structure of DNA, the chemical blueprint of life - when young men in the early Fifties. The work earned them a share of a Nobel prize and the universal respect that goes with it.

Free will began to interest Crick after he received a letter in 1986 from an old friend who wanted him to attend a discussion on the subject. Crick formulated a theory of free will based on a number of assumptions, including one that part of the brain is concerned with making plans for future actions without necessarily carrying them out. An important component of free will, he concluded, was that it must be able to personify behaviour - in other words, have an image of itself.

'And there I was content to let the matter rest,' Crick writes in his book An Astonishing Hypothesis. Until he stumbled across an account in the medical literature of a brain-damaged woman who was unresponsive; although she could follow people with her eyes, she did not speak spontaneously.

After largely recovering a month later, the woman related how upset she was at not being able to communicate. She had not talked because she felt she had nothing to say because her mind was empty.

Crick immediately thought she had lost her will, and later discovered the same idea had crossed the mind of the medical scientist who had studied her condition. The damage to her brain had occurred in a region called the anterior cingulate sulcus. Crick was delighted to hear that this part of the brain received nervous input from higher sensory regions and fitted neatly into his theoretical model of what sort of structure in the brain would control free will.

Like many novel developments in science, the idea had its first airing at afternoon tea. This informal gathering took place in the Salk Institute in San Diego, where Crick has worked since moving to the US. Crick relates the occasion in his book: 'I went over to tea one day and announced . . . that the seat of the Will had been discovered. It was at or near the anterior cingulate.'

Francis Crick was born in 1916, in the middle of the First World War. His parents were a middle- class couple who lived in Northampton. After a second-class degree in physics at London University, Crick set about postgraduate research on what he describes as the 'dullest problem imaginable', the determination of the viscosity of water, under pressure, between 100C and 150C. The Second World War saved him from terminal boredom by giving him a job in the Admiralty designing mines that would sink enemy shipping without exploding during a minesweeping operation.

When the war came to an end, Crick eventually decided there were two main topics he was interested in: the borderline between the living and the non-living, and the workings of the brain. He chose the former and began to study the structure of biological molecules under Max Perutz and Sir Lawrence Bragg at the Cavendish Laboratory in Cambridge.

By this time, Crick was no spring chicken. He was already 30, married for a second time, and had still not completed his doctorate. He soon met a brash young American with a crew cut - Jim Watson. They hit it off immediately, partly, Crick says, because they shared a youthful arrogance and impatience with sloppy thinking.

Indeed, Watson's controversial account of their discovery, The Double Helix, published in 1968, starts with the sentence 'I have never seen Francis Crick in a modest mood.' After the book came out, Crick confesses that he 'was a bit cross' but not about the personal comment. He was cross with Watson for other reasons: 'In those days, I didn't understand the business of conveying scientific ideas to the public and I thought he had grossly oversimplified a lot of the science. When I reread it later, I realised how cleverly he had done it.'

Forty-one years after the discovery was published, Crick is almost blase about what was done. The double helix, he says, was just waiting to be discovered and it was fortunate that he and Watson, along with Maurice Wilkins and Rosalind Franklin at King's College in London, got there first.

Crick's more recent research into conciousness and the brain fulfils his other early ambition. He is particularly interested in vision. Crick says there is far more to it than processing the light that triggers the thousands of sensors in the eye. Each eye, for instance, has a blind spot which should result in a small gap in our image of the world. The brain, however, ensures a continuity that should not really be there.

By understanding vision, and the way the brain works, we should come closer to discovering the soul, he says. 'I'm talking about the literal meaning of the word 'soul', roughly the religious one, something that intrinsically can't be explained scientifically. Of course, there is the metaphorical soul, one's inner feelings and that sort. We're not talking about that. The question is whether the literal one exists.' Crick is ready to be accused of being the arch reductionist for attempting to portray the most emotional aspects of humanity as the result of a pack of brain cells. Is the human soul nothing more than this? 'The answer is that we do want to reduce it to a bundle of neurons because we think that is all there is. But that doesn't mean to say that they aren't behaving in really complicated ways.'

If the soul is a mere collection of electrical activity in our heads, where does that leave religion? Crick lost faith in Christianity early in life and puts this down to a growing attachment to science. 'I realised early on that it is a detailed scientific knowledge which makes certain religious beliefs untenable,' he says.

Crick is in little doubt that science will one day understand human conciousness and free will in the same way we now understand the molecular genetics his discovery helped to reveal. It would be 'mildly suprising', he says, if this did not lead us to think differently about ourselves.

But there is one problem about the brain that he believes will elude science for ever. 'We will never understand what philosophers call 'qualia' - that is the painfulness of pain and the redness of red.' At least there will something in our heads that will remain a mystery.

'An Astonishing Hypothesis - The Scientific Search for the Soul', by Francis Crick, is published by Simon & Schuster at pounds 16.99.